1. Field of the Invention
The invention relates to a nuclear magnetic resonance method of detecting and monitoring the flocculation kinetics of high molecular weight fractions of a complex fluid.
2. Description of the Prior Art
Flocculation and deposition processes pose considerable problems in the petroleum industry. In particular for heavy oils, components of very high molar mass (asphaltenes, resins) are often the cause of such processes which may appear in porous media during production as well as during transportation. Flocculation is the formation of molecular aggregates of micronic size leading to sedimentation or deposition that can considerably modify the fluid flow, either by reduction of the section of flow or by viscosity increase. Furthermore, the intrinsic charge of some components (for example asphaltenes) generates a high tendency to cling to the charged surfaces.
The thermodynamic parameters which govern the flocculation processes are numerous (composition, pressure, temperature) and the complexity of the molecular structures involved make prediction and modelling very uncertain. Similarly, certain recovery methods (CO2 injection, acidizing) may modify the fluid equilibria and bring about these processes. It is thus necessary to carry out measurements but the available techniques do not allow the first stages of the flocculation process to be observed and pose considerable implementation problems as regards pressure and temperature or in-situ problems in oil wells.
Many petroleum crudes, notably those referred to as asphaltene crudes, are liquid hydrocarbon fluids which contain more or less large amounts of heavy fractions in the dissolved state and/or in the stable colloidal state in the pressure and temperature conditions to which the fluids are subjected. When these pressure and/or temperature conditions vary, notably when the pressure decreases, the heavy fractions contained in these fluids tend to flocculate and to settle in the formation in the neighbourhood of wells, in wells and in production and transfer facilities intended for the fluids. Thus, when a hydrocarbon reservoir containing heavy fractions is developed, generally before the bubble point is reached, the stability of these fractions decreases. When the saturation threshold is reached, the heavy fractions flocculate and settle, which can cause clogging of the porous media and formation of plugs likely to severely damage production wells and surface installations.
For oil producers whose task is to extract and convey, through production wells and pipe networks, liquid hydrocarbon fluids of petroleum crudes containing heavy fractions, for example asphaltene crudes, from production fields, it is therefore important to have precise knowledge of the pressure thresholds below which the heavy fractions settle, so as to carry out production and transfer of the fluids under pressure and temperature conditions preventing deposition of the heavy fractions in installations or to provide a suitable treatment.
Various methods of determining the deposition threshold of heavy fractions, notably asphaltenes, contained in liquid hydrocarbon fluids of petroleum crudes are known. These methods are most often optical light transmission or diffusion methods, conductimetric methods or viscosimetric methods.
A method known as the “spot test” deposits a small quantity of a mixture on a filter paper and observes the spot that forms. The flocculation aggregates that form in a mixture diffuse less readily than the surrounding liquid. Thus, if the spot is not uniform, it is an indication that it contains flocculating particles.
The aforementioned methods use detection of the variation of a physical quantity, for example an absorption coefficient or absorbance of light rays in the visible range or in the infrared range, electrical conductivity or viscosity, which results from the change in the structure of the fluid following flocculation and deposition of heavy fractions.
A major drawback of such methods is that they are not very selective insofar as it is not always easy to relate the variation of the physical quantity measured to the flocculation and deposition of heavy fractions, and these methods are not always sensitive to the deposition of a small amount of such fractions. Some methods, such as absorbance measurement in the infrared range, are very sensitive but difficult to implement under reservoir conditions.
Furthermore, since these methods are often used in the laboratory, the question which has to be considered is the representativity of the samples on which the physical quantity measurements are carried out. In fact, for a sample to be representative of the sampled fluid, it is necessary to maintain this sample under the pressure and temperature conditions that prevail for the sampled fluid, for example reservoir fluid, throughout the sampling, sample transport and storage operations preceding the measurements.
French patent 2,818,753 filed by the assignee describes a method of determining the deposition threshold of heavy fractions contained, in the dissolved state and/or in the stable colloidal state, in a liquid hydrocarbon fluid. The invention provides a method of determining the deposition threshold of heavy fractions, notably asphaltenes, contained in the dissolved state and/or in the stable colloidal state in a liquid hydrocarbon fluid, using the formation of an increasingly high pressure drop linked with the flow, at an increasing flow rate, of a sample of the fluid through a capillary passage. The fluid sample being in the dissolved state and/or in the stable colloidal state, at the inlet of a capillary passage a pressure drop which is at least equal to the difference between the pressure of the fluid sample and the bubble-point pressure of said sample is generated between the inlet and the outlet. A significant shift in the variation as a function of time of ΔP (difference between the pressure of the fluid at the capillary inlet and the pressure at the outlet) and of a quantity D representative of the flow of liquid flowing through the capillary passage is detected, which allows to characterize the deposition threshold of the heavy fractions of the fluid.
Besides, it is well-known that NMR devices can be used notably to measure certain physical characteristics of fluid mixtures such as hydrocarbons, notably the viscosity or the gas/oil ratio (GOR). The viscosity of the mixture and its GOR coefficient are obtained from the NMR measurement of a diffusion coefficient D and from the measurement of the longitudinal T1 or transverse T2 relaxation time. Such an application is for example described in WO-0,142,817 or U.S. Pat. No. 5,696,448, or in the following document:    Prammer, M. G. et al., (2001);“the Downhole NMR Fluid Analyser”; SPWLA 42nd Annual Logging Symposium.
It can be noted that these measurements are generally performed in a time interval after excitation which is unsuited to detection and interpretation of flocculation phenomena.
It is also well-known to use NMR devices to detect and monitor a very different phenomenon which is the crystallization of particles in fluids.